In recent years, various kinds of power conversion systems, each accommodating power between distributed power sources represented by solar power, wind power, or the like and a power system, have been interconnected in quantity to the power system. Thus, there is a concern that when a failure occurs in the power system, these power conversion systems may be disconnected simultaneously to cause a large power failure. Therefore, the power conversion system is required to include a function of continuous operation at a system failure. The function of continuous operation at a system failure refers to a function that, when a voltage of the power system is increased or decreased, or a frequency of the power system is increased or decreased, a power conversion system maintains a connection with the power system for a predetermined time.
There is a concern that, when the power system is stopped due to a failure in the system, these power conversion systems may have a state of isolated operation to continuously supply power to local system loads. It is because if the state of isolated operation is left untreated, workers may suffer electric shock or a restoration work of the power system may be disturbed. The power conversion system connected to the power system is required to include a device for detecting a state of isolated operation in order to prevent an isolated operation, and in case of transitioning to the state of isolated operation, to stop its operation quickly.
Japanese Patent Application Publication No. 2009-011037 and Japanese Patent Application Publication No. 2009-044910 disclose an exemplary method for detecting the isolated operation, namely a frequency shift method with step injection.
Japanese Patent Application Publication No. 2009-011037 describes in paragraph 0009 under Solution to Problems “A method for detecting an isolated operation according to the present invention, that is, the method for detecting an isolated operation which injects reactive power to the power system for the detection of whether or not a distributed power supply is disconnected from the power system to operate in an isolated state, is characterized that, when a system voltage is changed while in a state of no substantial change in the system frequency over a plurality of system cycles in the past, reactive power is injected into the system.”
Japanese Patent Application Publication No. 2009-011037 describes in paragraph 0010 “When two conditions: first, there is no substantial change in the system frequency over a plurality of system cycles in the past; and second, the system voltage fluctuates if an isolated operation occurs, are satisfied, reactive power is injected to actively break a state of balance with respect to the above-described reactive power, and then, as a result, power fluctuation is caused in the power system to allow an isolated operation to be surely detected.”
Japanese Patent Application Publication No. 2009-044910 describes under Solution to Problems “a first step that, when an isolated operation occurs, defines in advance a harmonic change pattern of harmonics changing over a plurality of system cycles in the past; a second step that determines whether or not measured harmonics exhibit changes corresponding to the harmonic change pattern; and a third step that, when it is determined that the measured harmonics exhibit changes corresponding to the harmonic change pattern, deems that harmonic fluctuation exists to inject reactive power, - - - .”
Thus, the frequency shift method with step injection increases, in a positive feedback manner, a command value of the reactive power according to the frequency deviation to activate fluctuation in the frequency. With a power conversion system using the frequency shift method with step injection, the frequency does not fluctuate when it is connected to the power system, while the frequency fluctuates in an isolated operation. Thus, the power conversion system can determine whether or not operating in an isolated mode.
Non-Patent Document of “New energy technology research and development: Technology research and development on interconnection test of multiple devices for detecting Isolated operation”, New Energy and Industrial Technology Development Organization, Feasibility Studies and Demonstration Projects H20 (2008) to H21 (2009), March in H 22 (2010), online, searched on Sep. 30, 2013, describes in page 90 “Frequency feedback function is a function in which reactive power is injected so as to encourage frequency changes. A first stage gain and a second stage gain are provided so as not to inject large reactive power at slight fluctuation in the frequency during a normal operation, however, when reactive power is injected up to near plus or minus 1 kvar in the second stage gain, the frequency changes much. (snip) For a first specimen, detection of an isolated operation is performed using four detection threshold values.”
As shown in FIG. 4.1.1-19 in the above mentioned Non-Patent Document, multiple threshold values are provided for the frequency deviation. In this algorithm, a time is measured after the deviation exceeds the threshold value, and if the deviation exceeds the threshold value for a predetermined time, it is determined that an isolated operation is being performed.